The potential of genomic DNA is realized when new modalities are invented that manipulate large DNAs with minimal breakage or loss of sample. Here, we describe a polydimethylsiloxane–polycarbonate membrane device to remove small molecules from a sample while retaining large DNAs. Dialysis rates dramatically change as DNA size in kb (M) increases and DNA dimensions become comparable to pore size, and chain characteristics go from rod-like to Gaussian. Consequently, we describe empirical rates of dialysis, R, as a function of M as falling into two regimes: DNAs ≤ 1 kb show R(M) ∼e - t/τ M (t = time, τM = time constant), while DNAs ≥1.65 kb slowly passage with R(M) ∼M -1.68; such partitioning potentiates single-molecule imaging.